The Air Accidents Investigation Branch said in an interim report that the right engine on the Eurocopter EC135 Type 2 helicopter "flamed out and shortly after the left engine flamed out" as it flew over the Scottish city on Nov. 29.

It said further investigation would try to determine why both engines failed when the helicopter had 168 pounds (76 kilograms) of fuel in its tanks.

It will also seek to understand why the helicopter failed to send an emergency signal and why it was unable to make a controlled descent when its engines failed.

Investigators said the aircraft's rotors were not turning when it slammed into the Clutha Vaults pub with such force that the roof collapsed. Three crew members and seven people on the ground were killed.

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